Best Practice for Risk Based Inspection

AE can be a very sensitive test method and has unique advantages in that:
• It generally surveys the whole structure under test.
• It does not require full access.
• Only registers the presence of ‘active’ defects.
However, it also has the disadvantage that it is very difficult to justify in
comparison with conventional NDT techniques applied with full access. It is vital
that there is confidence in the use of AE (resulting from experience of similar
applications), particularly as the test is dynamic and cannot easily be verified by
repetition. In general, it is not recommended that AE is used as the sole method of
inspection unless there is rigour in justification.
8.2.7. Magnetic Flux Leakage
Magnetic flux leakage (MFL) is only applicable to ferromagnetic materials. The
component is magnetised, and, depending on the level of induced flux density,
magnetic flux leakage due to both near and far surface flaws is detected by the
voltage induced in a detector coil, or Hall-effect sensor, traversed over the surface
of the component. Unlike MPI, the method is not limited to surface breaking or
near-surface flaws, but actually becomes increasingly sensitive to far surface flaws
with increasing levels of magnetisation. The output from the detector can be
amplified, filtered, digitised, stored etc. to produce an automated inspection system.
Multiple-element and differential probes are also used, and inspection speeds can be
very high.
MFL is finding increasing use in the petrochemical industry for providing highspeed
inspections of storage tank floors and carbon steel piping. These systems use
either permanent or electromagnets to provide localised near-magnetic saturation
coupled with induction coil or Hall-effect sensor arrays for detecting anomalous
flux leakage caused by corrosion defects (both near as well as far surface). Many of
these systems rely upon the use of an adjustable threshold, or amplitude gate, for
detection of corrosion in real-time. Some more advanced systems, through the use
of computerised equipment with signal processing, provide corrosion maps of
inspected areas similar to the C-scan presentation of ultrasonic data.
8.3. ASSESSMENT OF INSPECTION PERFORMANCE AND RELIABILITY
The normal process of inspection aims:
• To detect and locate areas of deterioration or flaws of concern.
• To determine their extent by providing flaw dimensions.
• To determine the type, or character, of flaws.
Inspection performance may be defined as the ultimate capabilities of an NDT
technique to detect, size and determine the type of a flaw in a given component.
Inspection reliability is a statistical measure of the expected variability in inspection
performance in many applications of the technique.
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